In certain tractors fitted with a CVT, in particular utility or compact tractors, the engine is controlled by means of both a hand lever and a foot lever, the latter being also referred to herein as a pedal. The engine operating point is chosen to correspond to the higher of two speed values demanded by the positions of the hand and foot levers. In normal operation, the engine operating point remains substantially constant and the wheel speed of the vehicle is varied by setting the transmission ratio in dependence upon the position of the foot lever. As the foot lever is depressed, the transmission ratio is increased to increase the wheel speed and this in turn increases the engine load. As long as the engine output power is capable of meeting the torque demand, the engine speed does not change appreciably when the foot pedal demands a lower engine speed than the hand lever, otherwise pressing the pedal results both in an increase of the transmission ratio and in an increase in the engine speed.
If, however, the vehicle is subjected to a sudden increase in wheel load, such as if the vehicle commences to travel up an incline, and the engine output power cannot be increased to meet the additional torque demand, then the engine speed will decrease and wheel speed will drop. The natural tendency of a driver under such conditions is to depress the foot lever still further, i.e. to treat the foot lever in the same way as an accelerator pedal and to accelerate harder. This, however, has an opposite effect to that desired by the operator, in that the demanded increase in transmission ratio will add to the engine load and cause the engine speed to drop further. This can ultimately result in stalling of the engine and the vehicle.